1. Field of the Invention
The present invention relates to fiber optic interferometers. More particularly, this invention pertains to a method and apparatus for increasing the long-term operational reliability of a fiber optic interferometer.
2. Description of the Prior Art
In known fiber optic systems for optical interferometric measurement of such physical quantities as, e.g., rate of rotation, an electrical voltage or current, the relative phase of two component beams which originate from a common light source and traverse the interferometer is affected by the physical quantity to be measured. The two component beams are brought into interference. The intensity of the interference beam produced is proportional to the phase shift between the two component beams. The interference beam is fed to a photodetector, forming an interference pattern thereon. The detector generates a detector output signal that is proportional to the intensity of the interference pattern, on the basis of which the physical quantity is determined. See, for example WO 99/04222 and DE 198 08 617 A1.
FIG. 4 is a schematic diagram a closed-loop fiber optic interferometer. A brief explanation follows with reference to FIG. 4 of the design and mode of operation of a Sagnac interferometer in a closed loop arrangement for measuring rate of rotation.
Light from a light source 21, such as a superluminescent diode (SLD), stabilized with regard to intensity, passes via a fiber link to a first beamsplitter 22, also denoted as a coupler. From there the light passes via a polarizer 23 to a second, main beamsplitter 24. The component beams produced by beam splitting pass to the two inputs/outputs of a fiber coil 25. A first phase modulator 26 is arranged between the outputs/inputs of the main beamsplitter 24 and those of the fiber coil 25. The component beams interfering in the main beam splitter 24 traverse the polarizer 23 after traversing the fiber coil 25, and are directed (as far as possible, to the extent of a half portion), to a photodetector 27 via the first beamsplitter 22. The output signal from the detector 27 is enhanced by an amplifier 28 and applied to a demodulator 29 and to a synchronous demodulator 30. The demodulator 29, along with an amplifying filter 31, form a scale factor controlled system.
The synchronous demodulator 30 utilizes a filter 32 to drive a ramp generator 33 that serves to generate a reset signal. The signal, generated by a modulation oscillator 34, for shifting the operating point to that of greatest sensitivity, and the reset signal are combined by an adder 35 to form a single signal that is input to a controllable amplifier 36. The amplifier 36 amplifies this signal with the aid of an amplification factor output of the amplifying filter 31. The output thereby obtained from the controllable amplifier 36 serves to drive the first phase modulator 26, which modulates and resets the optical phase correspondingly. It is therefore possible to measure a phase shift that is caused by rotation of the coil 25 (and is experienced by, the oppositely running component beams) due to the Sagnac effect. Moreover, the rate and magnitude rotation can be inferred.
In order to obtain a reliable detector output signal, and thus to permit reliable measurement of the physical quantity, the noise of the detector 27, or the noise in the detector output signal generated by the detector 27, should be as low as possible. Such noise is a function, inter alia, of the signal amplitude of the light beam striking the detector 27. Increasing the detector light power (i.e. increasing the intensity of the light beam striking the detector 27) reduces the noise of the detector output signal as the signal-to-noise ratio is thereby increased. The relationship between detector light power and noise in the detector output signal constitutes a variable characteristic of the detector 27 (or of the operating point of the detector 27) that can be determined by testing or calibration and is a measure of the detectivity of the detector 27.
As mentioned, the intensity of the light beam striking the detector 27 affects the noise in the detector output signal. Light beam intensity at the detector is, for the most part, a function of “path losses” suffered by the light during passage through the interferometer (e.g., in the beamsplitter 22, the polarizer 23 and the fiber coil 25). Light beam intensity is, however, also a function of the optical power (and/or fluctuations in the optical power) of the light source 21. As a result, the aging-induced, inherent decline of the optical light source power has a substantial influence on the noise and, thus, on the long-term reliability of the interferometer 20. Since the decline in light source power is unavoidable, it is known for the light source power to be set disproportionately high to insure sufficient light beam intensity even in the case of declining light source power. However, this has the disadvantage that loading of the light source 21, enlarged unnecessarily, leads to reduction of the useful life of the light source 21.